This invention relates to a moisture curable polyurethane hot melt adhesive and to a method of bonding substrates using the adhesive. In particular, this invention relates to a one component moisture curable polyurethane hot melt adhesive useful for bonding low surface energy substrates such as difficult-to-bond acrylonitrile-butadiene-styrene (ABS) substrates.
Moisture curable polyurethane hot melt adhesives are substantially solvent-free solids at room temperature that bond after application in the form of their melts by cooling and curing by the chemical reaction of isocyanate groups with moisture. Upon curing, the molecule size increases and the adhesive obtains its final characteristics.
Although suitable for many applications, moisture curable polyurethane hot melt adhesives have demonstrated limited ability to bond to certain low surface energy substrates, in particular difficult-to-bond ABS. ABS polymers are elastomeric and thermoplastic composites that exhibit excellent toughness. This property allows ABS to be used in a variety of applications making it one of the largest selling thermoplastics. A more complete description of the properties of ABS may be found in the Encyclopedia of Polymer Science and Engineering, Vol. 1:388-426 (Wiley, 1985).
Because of differences in its composition and production, not all grades of ABS are alike. While some ABS substrates are relatively easy to bond with moisture curable polyurethane hot melt adhesives, those skilled in the art generally recognize that many ABS substrates, referred to herein as difficult-to-bond ABS, are a challenge for polyurethane hot melt adhesives. For example, ABS substrates sold by Spartech Plastics, La Mirada, Calif. and Gage Plastics, Lake Oswego, Oreg. cannot be bonded with conventional moisture curable polyurethane hot melt adhesives.
While the reason for this shortcoming is not entirely clear, it is believed that the low surface energy of Spartech or Gage ABS renders it extremely difficult to bond with conventional polyurethane hot melt adhesives. Another possibility is that oil, mold release agent, and other possibly low molecular weight contaminants which may be used in the production process of ABS contribute to the difficulty of bonding ABS with polyurethane hot melt adhesives.
Thus, a moisture curable polyurethane hot melt adhesive that bonds to low surface energy substrates such as difficult-to-bond ABS, polyethylene, polypropylene, aluminum, and steel, as well as other substrates such as fiber-reinforced plastic, plywood, paper board, and the like, is highly desired.